1. Field of the Invention
The present invention relates to a connecting board disposed between a base plate such as a BGA type integrated circuit package base having surface-bonding terminals and a mounting board such as a mother board to which the base plate is bonded. The present invention further relates to a method of making such a connecting board. The present invention still further relates to an assembly of a base plate, a connecting board and a mounting board, and a subassembly of a base plate and a connecting board.
2. Description of the Prior Art
As the integrated circuit technology has been developed recently, the number of the input/output terminals provided to IC chips has also increased, and this has increased the number of the input/output terminals formed on an IC-mounted base plate on which the IC chips are mounted. However, it is not desired that the input/output terminals be formed on the peripheral portion of the base plate because the size of the base plate is enlarged as the number of the terminals increases whereby the cost of the IC-mounted base plates is elevated and their yield is lowered.
There has been widely used a so-called PGA (pin grid array) type base plate in which pins are arranged so as to form a grid pattern or a checkered pattern on the main surface (the main plane) of the IC-mounted base plate. However, the PGA type base plate has limitation when the number of the terminals is increased more and its size is decreased more.
The next method has been adopted to improve this situation. In place of the pins, pads (or lands) are arranged so as to form a grid pattern or a checkered pattern on the surface of the base plate, and bumps, to which terminal members made of substantially ball-like high temperature solder or a metal such as copper or silver having good solder wettablility is eutectically soldered, are formed on the pads. Other pads are formed on the portions of the related printed circuit board (PCB) such as a mother board at the positions corresponding to the pads of the IC-mounted base plate, and eutectic solder paste is applied to the pads of the printed circuit board. Thereafter, both the base plate and the printed circuit board are piled up and heated to melt the solder paste. In this way, the base plate and the printed circuit board are bonded together by solder through the terminal members. In general, a base plate which is provided with only pads arranged so as to form a grid pattern is called an LGA (land grid array) type base plate and a base plate which is provided with ball-shaped terminal members on the pads is called a BGA (ball grid array) type base plate.
When terminals such as pads or bumps are formed so as to constitute a grid pattern (including a checkered pattern) on the planar surface of an IC-mounted base plate and/or a printed circuit board and the IC-mounted base plate is bonded to the printed circuit board (hereinafter such bonding being referred to as the "surface bonding"), difference in thermal expansion occurs between the IC-mounted base plate and the printed circuit board along the planar surface because the coefficients of thermal expansion of the IC-mounted base plate and the printed circuit board due to the difference in their materials. In other words, the IC-mounted base plate and the printed circuit board which are bonded together tend to deform in the same directions along the planar surface but by a different amount from each other so that shearing stress is exerted on the terminal members and the pads.
The shearing stress becomes maximum between the two terminals which are disposed remotest from each other when the magnitude of the shearing stress is taken into account between any two surface-bonded terminals. More specifically, in case the terminals are arranged so as to form a grid pattern and the outermost terminals form a square, the largest difference in thermal expansion occurs between the outermost terminals at each pair of diagonal corners of the square, and the largest shearing stress is exerted on each pair of the outermost terminals. Upon bonding a board such as an LGA type base plate or a BGA type base plate to a printed circuit board in particular, the interval (pitch) between the terminals is relatively large, tending to make the distance between the most separated terminals large. Particularly, when an LGA type or BGA type base plate is made of ceramic, the base plate has a smaller coefficient of thermal expansion than a printed circuit board generally made of glass epoxy resin, and thus the board is applied with a larger shearing stress.
When such shearing stress is applied to the IC-mounted base plate in case the adhering strength (bonding strength) between the pads formed on the IC-mounted base plate and the solder is not so large, bonding between the board and the solder is sometimes broken. I.e., the solder as well as the terminal members are sometimes removed from the pads. Thus, it is desired to make the adhering strength large enough.
When, however, the adhering strength between the pads and the solder is increased, cracks extending substantially in parallel with pads are produced in the solder in the vicinity of the related pads due to repeated thermal stress, and, finally, the solder is broken. Thus, it is impossible to attain high bonding reliability. Since the solder in the vicinity of the pads is generally of a eutectic type, it is relatively hard and brittle and a secular change in the solder occurs easily due to heat and stress. Owing to this nature, cracks are produced in the solder when repeated stress is applied.
This problem is likely to be arisen between an LGA type base plate (or a BGA type base plate) made of ceramic which has a relative low coefficient of thermal expansion and a printed circuit board made of resin such as glass epoxy resin which has a relatively high coefficient of thermal expansion. In most cases, cracks are produced eutectic solder portions in the vicinity of the pads formed on the ceramic base plate, because the base plate made of ceramic is hard and does not absorb stress whereas the printed circuit board made of resin and the pads made of copper or the like are relatively soft and absorb the stress.
Japanese patent application provisional publication No. 8-55930 discloses a package for housing a semiconductor, having pads disposed on the bottom of a depression formed in the lower surface of an insulating substrate and ball-shaped terminal members having a predetermined dimensional relation to the pads and soldered to the pads. In this arrangement, the ball-shaped terminals are brazed to the pads accurately and securely.
In the package of this prior art, however, the depression must be formed in the insulated base plate (IC-mounted base plate) and pads must be provided on the bottoms of the depression, leading to a complicated structure. This makes it difficult to fabricate the package and elevates its manufacturing cost. It is also difficult to insert a blazing material in the depression and to blaze the ball-shaped terminals to the pads.
The LGA type base plate is bonded to the printed circuit board in the following cumbersome steps. Ball-shaped terminal members made of solder having a high melting point or copper are temporarily fixed to the lands (pads) of the LGA type base plate by means of solder paste such eutectic solder paste having a lower melting point than the terminal members. Then, reflow is conducted to solder the terminal members to the pads and a BGA type base plate is fabricated. Here, the solder having a low melting point and the solder having a high melting point are hereinafter referred to as the "low temperature solder" and the "high temperature solder", respectively. After low temperature solder paste has been applied to the pads on the printed circuit board, the BGA type base plate is mounted on the printed circuit board and the terminal members are aligned with the pads on the printed circuit board. Thereafter, reflowing is performed again to solder the terminal members to the pads on the printed circuit board.
After having purchased LGA type base plates on which IC chips are to be mounted, having mounted the IC chips on the base plates and having performed flip chip bonding, IC chip makers or manufacturers must bond the terminal members to the pads (lands) of the base plates by means of low temperature solder (for example, eutectic solder) which has a melting point lower than the solder (for example, high temperature solder) used for the flip chip bonding. In addition to the equipment or apparatus for flip-chip bonding the IC chips to the base plates, there are required apparatuses for fabricating a BGA type base plate from an LGA type base plate, i.e., apparatuses for bonding the terminal members to pads, such as an apparatus for applying solder paste (for example, eutectic solder paste) to the pads and an apparatus for mounting the terminal members on the pads.